Raman radiation results from inelastic scattering. When the monochromatic excitation radiation is directed to the object material, low-energy modes, such as vibration and rotation of molecules cause small deviations in the wavelength of the monochromatic radiation. Each deviation, in turn, is characteristic to each molecule in the material and hence molecules in the material can be identified.
The Raman radiation is notoriously difficult to measure since its intensity with respect to the excitation radiation is very low and it arrives at the detector almost simultaneously with the excitation radiation. Additionally, the excitation radiation gives rise to fluorescent radiation which is also simultaneous with the Raman radiation and whose lifetime is in nanosecond range.
Notch filters are usually employed to block the excitation radiation away as effectively as possible without attenuating other wavelengths excessively. The Raman radiation has also been separated from the fluorescent radiation using a gating device in front of the detector. For example, an optically controlled Kerr-gate may be placed in front of a detector. The Kerr-gate passes through optical radiation when it is in a transparent state and it blocks optical radiation when it is in a non-transparent state. The Kerr-gate can be switched to the transparent state using an optical control pulse from a laser, for example, and the Kerr-gate remains in the transparent state for the duration of the optical control pulse. At the end of the optical control pulse, the Kerr-gate returns to the non-transparent state. The Kerr-gate can be switched to the transparent state repeatedly by the optical control pulse for a desired period of time with an inaccuracy of a few picoseconds. In that way, both the excitation radiation and the fluorescent radiation can be suppressed effectively and the Raman radiation can be detected.
Instead of Kerr-gate, an image intensifier may correspondingly be placed in the front of the detector such as a CCD camera (Charge Coupled Device). The image intensifier which may also be called a wafer tube or a proximity-focused intensifier operates as a photomultiplier having more than a thousand volt over it. In addition to amplifying the received radiation, the image intensifier can be switched on and off with a frequency in a megahertz range and with a gate period of several hundreds on picoseconds.
However, there are problems related to the prior art. The duty cycle of a Kerr-gate is low, since the repetition rate of the transparent states of the Kerr-gate is typically less than one kilohertz, which makes the measurement unpractical. The operation of the Kerr-gate also needs high-energy optical pulses which drastically limit the energy of the optical pulses directed to the measured object from the same optical source. Correspondingly, the image intensifier has a problem due to a difficult and contradictious requirement of forming short pulses with well over 1000V. Background noise such as thermal noise and electron multiplication noise strongly limit the signal-to-noise ratio and disturb the measurements using the Kerr-gate and the image intensifier. Both measurement systems are also complicated, expensive and large such that they can only be used in laboratory.
Hence, there is a need for a better solution to measure Raman radiation.